Typically, release sheets are used for protecting adhesive layer surfaces such as labels prior to use, and consequently a release agent layer is provided on at least one surface of the substrate. Furthermore in an adhesive tape, an adhesive layer is provided on one surface of a substrate, and a release agent layer is provided on the other surface. Long chain alkyl group containing compounds, silicone modified resin based release agents or silicone based release agents are typically used as the release agent, and silicone based release agents are currently the most widely used as they exhibit excellent releasability relative to adhesives.
However, in the aforementioned silicone based release agents, the critical surface tension for the surface of the release agent layer is small, and writing on the surface of the release agent layer with a marker pen or conducting thermal transfer printing with an ink ribbon is impossible. Furthermore, the coefficient of friction is low, making the surface slippery, and stepping on a pile of stored release sheets can result in the person slipping and falling over, whereas in a rolled state, slippage in the width direction can cause finishing loss or loss within secondary processing. Furthermore, when used as an adhesive tape, because the back surface of the tape is slippery, if corrugated fiberboard boxes which have been packed using this tape are piled up, then the tape is likely to cause undesirable slipping or collapse within the pile of boxes.
Moreover, release sheets with a coating of a long chain alkyl group containing compound as a release agent also exist. A release agent layer using this type of long chain alkyl group containing compound has a high coefficient of friction for the release surface, and consequently is resistant to slipping, and can be drawn on with an oil based marker pen or the like. However, long chain alkyl group containing compounds also suffer problems in that the release force is too high when compared with silicone based release agents, and furthermore the heat resistance-is poor, so that if exposed to high temperature while bonded to an adhesive layer, the release layer becomes difficult to remove, causing abnormal release. Furthermore, almost all long chain alkyl group containing compounds display poor printing and print recordability using thermal transfer systems. In other words, ink transfer to the surface of the release agent by thermal transfer was not unsatisfactory, and the adhesion of ink to the release agent surface was almost non-existent.
In this manner, both silicone based release agents and long chain alkyl group containing compounds offer advantages and disadvantages and a release sheet with a release agent layer capable of resolving the shortcomings of both of the above types of compounds, namely a release sheet with excellent releasability, printing and print recordability, which is slip resistant (or non-slip) and offers superior heat resistance, as well as an adhesive tape which uses such a release sheet, have become keenly sought. Furthermore, when thermal transfer methods are used to attempt to print to the surface of a conventional release sheet or adhesive tape, either the ink does not transfer satisfactorily, or even if the ink does transfer, it is easily removed, making such printing impossible in practical applications. Consequently, a release sheet which combines releasability and printability, as well as an adhesive tape using such a release sheet have become keenly sought.
The inventors of the present invention have already proposed, in Japanese Unexamined Patent Application, First Publication No. Hei 11-228924, a release agent layer in which either the mass ratio is defined between a specific silicone and an organic resin with a glass transition point which falls within a specific range, or alternatively the mass ratio is defined between three essential components, namely, a specific silicone, a cellulose derivative and a reactive organic resin based control agent, and the contact angle of the surface of a release agent layer formed by applying a release agent comprising the aforementioned 2 components or 3 components, and the release force of an adhesive layer relative to the surface of the release agent layer are also restricted to a specific range of values. By providing this release agent layer, an adhesive tape can be obtained which can be written on, displays non-slip characteristics, and for which overlapping adhesion is possible.
However, in order to apply the invention proposed above and use it within a release sheet application, the proportion of each component in the blended release agent composition containing a polydimethylsiloxane and a polymer resin was altered to try and adjust the balance between the writability, the coefficient of friction and the release force and the like, but the desired performance could not always be obtained. For example, even if the proportion of the polymer resin was increased to try and impart writability, non-slip characteristics and a suitable level of releasability, in some cases, almost the same quality was obtained regardless of the polymer content. In addition, even if the same release agent recipe was used, when the release agent compositions were then applied to a substrate and dried, the resulting physical characteristics such as the writability, the coefficient of friction, and the level of release force relative to an identical adhesive could vary, and in some cases a uniform quality could not be obtained.
Furthermore, the inventors of the present invention have also proposed, in Japanese Unexamined Patent Application, First Publication No. 2000-73021, a thermal transfer printable adhesive tape wherein a synthetic resin layer is provided on one surface of an adhesive tape substrate, and the glass transition temperature (Tg) of the synthetic resin which comprises the main component of the synthetic resin layer is no more than −20° C. and the contact angle (measured using a water and ethanol solution with a 1:1 mass ratio, with measurement conducted 60 seconds after dropping a drop of volume 5 μl) of the synthetic resin layer surface is within a range from 25 to 70°, and an adhesive layer is provided on the other surface of the substrate. According to this invention, an adhesive tape and an adhesive sheet which required no release sheet, and enabled both good releasability from the adhesive, and good printing and print recordability using thermal transfer systems could be provided.
However, depending on the type of synthetic resin used in Japanese Unexamined Patent Application, First Publication No. 2000-73021, in those cases in which a coating layer such as a synthetic resin layer or a release agent layer was provided and the tape was then stored in a roll paper, blocking (a phenomenon in which stickiness develops) could develop between the surface of the applied layer and other surfaces, and the ink layer from the ink ribbon sometimes transferred to the surface of the release agent layer even though printing was not intended (hereafter this is referred to as ink soiling). This type of blocking and ink soiling become more marked under heat treatment or with the passage of time.
Accordingly, an object of the present invention is to provide a release sheet and an adhesive tape which display superior characteristics such as excellent releasability from an adhesive layer, excellent printing and print recordability for thermal transfer systems, little susceptibility to slipping when release agent layers are placed together, as well as little susceptibility to blocking, no ink soiling, and superior writability.